Current studies examining the transmission of oncoviruses across phylogenetic lines have demonstrated the productive transmission of the helper virus associated with avian reticulendotheliosis (REV-A) to cultured mammalian cells. The ability of ten mammalian derived cell lines to propagate REV-A has been examined. Only FCf2Th cells (a line from embryonic dog thymus) were found suitable for long term virus propagation, though transient virus replication also occurred in cells from mink lung. The virus propagated in dog cells (D-REV) was transmissible back to chick cells and baby chicks causing typical REV-A pathology. Comparison of D-REV to C-REV (a mixture of REV-A and oncogenic REV produced by transformed chick bone marrow cells) by immunogel diffusion, SDS-polyacrylamide gel electrophoresis, reverse transcriptase anti-serum neutralization and peptide mapping of the major core protein confirmed that D-REV closely resembled C-REV.